// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef NET_DNS_HOST_RESOLVER_IMPL_H_
#define NET_DNS_HOST_RESOLVER_IMPL_H_

#include <stddef.h>
#include <stdint.h>

#include <map>
#include <memory>

#include "base/macros.h"
#include "base/memory/weak_ptr.h"
#include "base/strings/string_piece.h"
#include "base/threading/non_thread_safe.h"
#include "base/time/time.h"
#include "net/base/net_export.h"
#include "net/base/network_change_notifier.h"
#include "net/dns/host_cache.h"
#include "net/dns/host_resolver.h"
#include "net/dns/host_resolver_proc.h"
#ifdef TENCENT_CHANGES
#include "tencent/build/config_net.h"
#endif

namespace net {

class AddressList;
class BoundNetLog;
class DnsClient;
class IPAddress;
class NetLog;

// For each hostname that is requested, HostResolver creates a
// HostResolverImpl::Job. When this job gets dispatched it creates a ProcTask
// which runs the given HostResolverProc on a worker thread (a WorkerPool
// thread, in production code.) If requests for that same host are made during
// the job's lifetime, they are attached to the existing job rather than
// creating a new one. This avoids doing parallel resolves for the same host.
//
// The way these classes fit together is illustrated by:
//
//
//            +----------- HostResolverImpl -------------+
//            |                    |                     |
//           Job                  Job                   Job
//    (for host1, fam1)    (for host2, fam2)     (for hostx, famx)
//       /    |   |            /   |   |             /   |   |
//   Request ... Request  Request ... Request   Request ... Request
//  (port1)     (port2)  (port3)      (port4)  (port5)      (portX)
//
// When a HostResolverImpl::Job finishes, the callbacks of each waiting request
// are run on the origin thread.
//
// Thread safety: This class is not threadsafe, and must only be called
// from one thread!
//
// The HostResolverImpl enforces limits on the maximum number of concurrent
// threads using PrioritizedDispatcher::Limits.
//
// Jobs are ordered in the queue based on their priority and order of arrival.
class NET_EXPORT HostResolverImpl
    : public HostResolver,
      NON_EXPORTED_BASE(public base::NonThreadSafe),
      public NetworkChangeNotifier::IPAddressObserver,
      public NetworkChangeNotifier::ConnectionTypeObserver,
      public NetworkChangeNotifier::DNSObserver {
public:
    // Parameters for ProcTask which resolves hostnames using HostResolveProc.
    //
    // |resolver_proc| is used to perform the actual resolves; it must be
    // thread-safe since it may be run from multiple worker threads. If
    // |resolver_proc| is NULL then the default host resolver procedure is
    // used (which is SystemHostResolverProc except if overridden).
    //
    // For each attempt, we could start another attempt if host is not resolved
    // within |unresponsive_delay| time. We keep attempting to resolve the host
    // for |max_retry_attempts|. For every retry attempt, we grow the
    // |unresponsive_delay| by the |retry_factor| amount (that is retry interval
    // is multiplied by the retry factor each time). Once we have retried
    // |max_retry_attempts|, we give up on additional attempts.
    //
    struct NET_EXPORT_PRIVATE ProcTaskParams {
        // Sets up defaults.
        ProcTaskParams(HostResolverProc* resolver_proc, size_t max_retry_attempts);

        ProcTaskParams(const ProcTaskParams& other);

        ~ProcTaskParams();

        // The procedure to use for resolving host names. This will be NULL, except
        // in the case of unit-tests which inject custom host resolving behaviors.
        scoped_refptr<HostResolverProc> resolver_proc;

        // Maximum number retry attempts to resolve the hostname.
        // Pass HostResolver::kDefaultRetryAttempts to choose a default value.
        size_t max_retry_attempts;

        // This is the limit after which we make another attempt to resolve the host
        // if the worker thread has not responded yet.
        base::TimeDelta unresponsive_delay;

        // Factor to grow |unresponsive_delay| when we re-re-try.
        uint32_t retry_factor;
    };

    // Creates a HostResolver as specified by |options|. Blocking tasks are run on
    // the WorkerPool.
    //
    // If Options.enable_caching is true, a cache is created using
    // HostCache::CreateDefaultCache(). Otherwise no cache is used.
    //
    // Options.GetDispatcherLimits() determines the maximum number of jobs that
    // the resolver will run at once. This upper-bounds the total number of
    // outstanding DNS transactions (not counting retransmissions and retries).
    //
    // |net_log| must remain valid for the life of the HostResolverImpl.
    HostResolverImpl(const Options& options, NetLog* net_log);

    // If any completion callbacks are pending when the resolver is destroyed,
    // the host resolutions are cancelled, and the completion callbacks will not
    // be called.
    ~HostResolverImpl() override;

    // Configures maximum number of Jobs in the queue. Exposed for testing.
    // Only allowed when the queue is empty.
    void SetMaxQueuedJobs(size_t value);

    // Set the DnsClient to be used for resolution. In case of failure, the
    // HostResolverProc from ProcTaskParams will be queried. If the DnsClient is
    // not pre-configured with a valid DnsConfig, a new config is fetched from
    // NetworkChangeNotifier.
    void SetDnsClient(std::unique_ptr<DnsClient> dns_client);

#ifdef ENABLE_TENCENT_DNS_CACHE
    void ClearDnsCacheAndDatabase() override;
#endif

    // HostResolver methods:
    int Resolve(const RequestInfo& info,
        RequestPriority priority,
        AddressList* addresses,
        const CompletionCallback& callback,
        RequestHandle* out_req,
        const BoundNetLog& source_net_log) override;
    void ChangeRequestPriority(RequestHandle req,
        RequestPriority priority) override;
    void CancelRequest(RequestHandle req) override;
    int ResolveFromCache(const RequestInfo& info,
        AddressList* addresses,
        const BoundNetLog& source_net_log) override;
    void SetDnsClientEnabled(bool enabled) override;
    HostCache* GetHostCache() override;
    std::unique_ptr<base::Value> GetDnsConfigAsValue() const override;

    // Like |ResolveFromCache()|, but can return a stale result if the
    // implementation supports it. Fills in |*stale_info| if a response is
    // returned to indicate how stale (or not) it is.
    int ResolveStaleFromCache(const RequestInfo& info,
        AddressList* addresses,
        HostCache::EntryStaleness* stale_info,
        const BoundNetLog& source_net_log);

    void set_proc_params_for_test(const ProcTaskParams& proc_params)
    {
        proc_params_ = proc_params;
    }

protected:
    // Just like the public constructor, but allows the task runner used for
    // blocking tasks to be specified. Intended for testing only.
    HostResolverImpl(const Options& options,
        NetLog* net_log,
        scoped_refptr<base::TaskRunner> worker_task_runner);

    // Callback from HaveOnlyLoopbackAddresses probe.
    void SetHaveOnlyLoopbackAddresses(bool result);

private:
    friend class HostResolverImplTest;
    class Job;
    class ProcTask;
    class LoopbackProbeJob;
    class DnsTask;
    class Request;
    typedef HostCache::Key Key;
    typedef std::map<Key, Job*> JobMap;

    // Number of consecutive failures of DnsTask (with successful fallback to
    // ProcTask) before the DnsClient is disabled until the next DNS change.
    static const unsigned kMaximumDnsFailures;

    // Helper used by |Resolve()| and |ResolveFromCache()|.  Performs IP
    // literal, cache and HOSTS lookup (if enabled), returns OK if successful,
    // ERR_NAME_NOT_RESOLVED if either hostname is invalid or IP literal is
    // incompatible, ERR_DNS_CACHE_MISS if entry was not found in cache and
    // HOSTS and is not localhost.
    //
    // If |allow_stale| is true, then stale cache entries can be returned.
    // |stale_info| must be non-null, and will be filled in with details of the
    // entry's staleness (if an entry is returned).
    //
    // If |allow_stale| is false, then stale cache entries will not be returned,
    // and |stale_info| must be null.
    int ResolveHelper(const Key& key,
        const RequestInfo& info,
        const IPAddress* ip_address,
        AddressList* addresses,
        bool allow_stale,
        HostCache::EntryStaleness* stale_info,
        const BoundNetLog& request_net_log);

    // Tries to resolve |key| as an IP, returns true and sets |net_error| if
    // succeeds, returns false otherwise.
    bool ResolveAsIP(const Key& key,
        const RequestInfo& info,
        const IPAddress* ip_address,
        int* net_error,
        AddressList* addresses);

    // If |key| is not found in cache returns false, otherwise returns
    // true, sets |net_error| to the cached error code and fills |addresses|
    // if it is a positive entry.
    //
    // If |allow_stale| is true, then stale cache entries can be returned.
    // |stale_info| must be non-null, and will be filled in with details of the
    // entry's staleness (if an entry is returned).
    //
    // If |allow_stale| is false, then stale cache entries will not be returned,
    // and |stale_info| must be null.
    bool ServeFromCache(const Key& key,
        const RequestInfo& info,
        int* net_error,
        AddressList* addresses,
        bool allow_stale,
        HostCache::EntryStaleness* stale_info);

    // If we have a DnsClient with a valid DnsConfig, and |key| is found in the
    // HOSTS file, returns true and fills |addresses|. Otherwise returns false.
    bool ServeFromHosts(const Key& key,
        const RequestInfo& info,
        AddressList* addresses);

    // If |key| is for a localhost name (RFC 6761), returns true and fills
    // |addresses| with the loopback IP. Otherwise returns false.
    bool ServeLocalhost(const Key& key,
        const RequestInfo& info,
        AddressList* addresses);

    // Returns the (hostname, address_family) key to use for |info|, choosing an
    // "effective" address family by inheriting the resolver's default address
    // family when the request leaves it unspecified.
    Key GetEffectiveKeyForRequest(const RequestInfo& info,
        const IPAddress* ip_address,
        const BoundNetLog& net_log);

    // Probes IPv6 support and returns true if IPv6 support is enabled.
    // Results are cached, i.e. when called repeatedly this method returns result
    // from the first probe for some time before probing again.
    virtual bool IsIPv6Reachable(const BoundNetLog& net_log);

    // Asynchronously checks if only loopback IPs are available.
    virtual void RunLoopbackProbeJob();

    // Records the result in cache if cache is present.
    void CacheResult(const Key& key,
        const HostCache::Entry& entry,
        base::TimeDelta ttl);

    // Removes |job| from |jobs_|, only if it exists.
    void RemoveJob(Job* job);

    // Aborts all in progress jobs with ERR_NETWORK_CHANGED and notifies their
    // requests. Might start new jobs.
    void AbortAllInProgressJobs();

    // Aborts all in progress DnsTasks. In-progress jobs will fall back to
    // ProcTasks. Might start new jobs, if any jobs were taking up two dispatcher
    // slots.
    void AbortDnsTasks();

    // Attempts to serve each Job in |jobs_| from the HOSTS file if we have
    // a DnsClient with a valid DnsConfig.
    void TryServingAllJobsFromHosts();

    // NetworkChangeNotifier::IPAddressObserver:
    void OnIPAddressChanged() override;

    // NetworkChangeNotifier::ConnectionTypeObserver:
    void OnConnectionTypeChanged(
        NetworkChangeNotifier::ConnectionType type) override;

    // NetworkChangeNotifier::DNSObserver:
    void OnDNSChanged() override;
    void OnInitialDNSConfigRead() override;

    void UpdateDNSConfig(bool config_changed);

    // True if have a DnsClient with a valid DnsConfig.
    bool HaveDnsConfig() const;

    // Called when a host name is successfully resolved and DnsTask was run on it
    // and resulted in |net_error|.
    void OnDnsTaskResolve(int net_error);

    // Allows the tests to catch slots leaking out of the dispatcher.  One
    // HostResolverImpl::Job could occupy multiple PrioritizedDispatcher job
    // slots.
    size_t num_running_dispatcher_jobs_for_tests() const
    {
        return dispatcher_->num_running_jobs();
    }

    // Cache of host resolution results.
    std::unique_ptr<HostCache> cache_;

    // Map from HostCache::Key to a Job.
    JobMap jobs_;

    // Starts Jobs according to their priority and the configured limits.
    std::unique_ptr<PrioritizedDispatcher> dispatcher_;

    // Limit on the maximum number of jobs queued in |dispatcher_|.
    size_t max_queued_jobs_;

    // Parameters for ProcTask.
    ProcTaskParams proc_params_;

    NetLog* net_log_;

    // If present, used by DnsTask and ServeFromHosts to resolve requests.
    std::unique_ptr<DnsClient> dns_client_;

    // True if received valid config from |dns_config_service_|. Temporary, used
    // to measure performance of DnsConfigService: http://crbug.com/125599
    bool received_dns_config_;

    // Number of consecutive failures of DnsTask, counted when fallback succeeds.
    unsigned num_dns_failures_;

    // True if DnsConfigService detected that system configuration depends on
    // local IPv6 connectivity. Disables probing.
    bool use_local_ipv6_;

    base::TimeTicks last_ipv6_probe_time_;
    bool last_ipv6_probe_result_;

    // True iff ProcTask has successfully resolved a hostname known to have IPv6
    // addresses using ADDRESS_FAMILY_UNSPECIFIED. Reset on IP address change.
    bool resolved_known_ipv6_hostname_;

    // Any resolver flags that should be added to a request by default.
    HostResolverFlags additional_resolver_flags_;

    // Allow fallback to ProcTask if DnsTask fails.
    bool fallback_to_proctask_;

    // Task runner used for DNS lookups using the platform resolver, and other
    // blocking operations. Usually just the WorkerPool's task runner for slow
    // tasks, but can be overridden for tests.
    scoped_refptr<base::TaskRunner> worker_task_runner_;

    base::WeakPtrFactory<HostResolverImpl> weak_ptr_factory_;

    base::WeakPtrFactory<HostResolverImpl> probe_weak_ptr_factory_;

    DISALLOW_COPY_AND_ASSIGN(HostResolverImpl);
};

// Resolves a local hostname (such as "localhost" or "localhost6") into
// IP endpoints with the given port. Returns true if |host| is a local
// hostname and false otherwise. Special IPv6 names (e.g. "localhost6")
// will resolve to an IPv6 address only, whereas other names will
// resolve to both IPv4 and IPv6.
// This function is only exposed so it can be unit-tested.
// TODO(tfarina): It would be better to change the tests so this function
// gets exercised indirectly through HostResolverImpl.
NET_EXPORT_PRIVATE bool ResolveLocalHostname(base::StringPiece host,
    uint16_t port,
    AddressList* address_list);

} // namespace net

#endif // NET_DNS_HOST_RESOLVER_IMPL_H_
